Tjaart Van der Walt fired a 65 of his own to finish alone in second place at 14-under-par 202. Ken Duke, who played alongside Couch on Saturday, was one shot further back at 13-under-par 203.
Couch had already put together two solid rounds on the Senator Course at Capitol Hill before taking control of the tournament on Saturday.
He ran off back-to-back birdies starting at the par-4 third but found trouble with a bogey at the fifth. Couch recovered with a birdie at the seventh and rolled in a six-foot putt for a birdie at the eighth.
At the par-5 10th, Couch hit a stellar second shot that stopped within inches of the cup. He tapped in for the eagle to reach 16 under.
'That might be the best shot of my life,' said Couch. 'When it was in the air I thought it was going in. I don't think I've ever hit a shot that high before.'
Couch stumbled with a bogey at the 12th but countered at the par-4 14th and his approach dropped inside 12 feet for a birdie. Couch kept on rolling and knocked his second shot to five feet for a birdie at the very next hole.
The 30-year-old two-putted for a birdie at the par-5 17th to cap the dominating performance.
'I told my coach earlier this week that this is the best I've ever swung it,' said Couch, who has already secured his PGA Tour card for 2004. 'I'm striking the ball really well and making putts. But I never imagined it would be 18 under par after three rounds. I just want to stay focused for 18 holes tomorrow.'
Van der Walt is in a slightly different position than Couch. The South African is currently 25th on the money list, but a strong finish on Sunday would be enough to move him into the top 20 and grant him an exemption on the PGA Tour.
He collected three birdies and a bogey on the front side before coming on strong down the stretch.
Van der Walt tallied two straight birdies from the 14th and dropped his second shot inside six feet en route to an eagle at the par-5 17th.
At the par-4 last, Van der Walt drained a long birdie putt to finish four shots off the pace.
'The putts I've been missing I made up for on the last few holes,' said Van der Walt. 'I didn't even look at the board so I don't know where I'm at. But I do know I played well today.'
Wes Short posted a 5-under 67 to join D.J. Brigman in a tie for fourth at 11-under-par 205. Nationwide Tour money leader Zach Johnson was one shot further back at 10-under-par 206 along with Jason Bohn, Daniel Chopra and Chris Tidland.
Jess Daley and Doug LaBelle II shared 10th place at 9-under-par 207.